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Ethanol metabolism

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Ethanol, an alcohol found in nature and in alcoholic drinks, is metabolized through a complex catabolic metabolic pathway.

Human metabolic physiology

Ethanol and evolution

The average human digestive system produces approximately 3g of ethanol per day through fermentation of its contents.[1] Catabolic degradation of ethanol is thus essential to life, not only of humans, but of almost all living organisms. Certain amino acid sequences in the enzymes used to oxidize ethanol are conserved (unchanged) going back to single cell bacteria.[2] Such a function is necessary because all organisms produce alcohol in small amounts by several pathways, primarily through fatty acid synthesis,[3] glycerolipid metabolism,[4] and bile acid biosynthesis pathways.[5] If the body had no mechanism for catabolizing the alcohols, they would build up in the body and become toxic. This could be an evolutionary rationale for alcohol catabolism also by sulfotransferase.

Physiologic structures

A basic organizing theme in biological systems is that increasing complexity in specialized tissues and organs, allows for greater specificity of function. This occurs for the processing of ethanol in the human body. The enzymes required for the oxidation reactions are confined to certain tissues. In particular, much higher concentration of such enzymes are found in the liver,[6] which is the primary site for alcohol catabolism. Variations in genes influence alcohol metabolism and drinking behavior.[7]

Thermodynamic considerations

Energy thermodynamics

Energy calculations

The reaction from ethanol to carbon dioxide and water is a complex one that proceeds in three steps. Below, the Gibbs free energy of formation for each step is shown with ΔGf values given in the CRC.[8]

Complete reaction: C2H6O(Ethanol)→C2H4O(Acetaldehyde)→C2H4O2(acetic acid) →Acetyl-CoA→3H2O+2CO2.

ΔGf = Σ ΔGfp − ΔGfo

Step One

C2H6O(Ethanol) + NAD+ →C2H4O(Acetaldehyde) + NADH + H+
Ethanol: −174.8 kJ/mol
Acetaldehyde: −127.6 kJ/mol
ΔGf1 = −127.6 + 174.8 = 47.2 kJ/mol(Endergonic)
ΣΔGf = 47.2 kJ/mol (Endergonic, but this does not take into consideration the simultaneous reduction of NAD+.)

Step Two

C2H4O(Acetaldehyde) + NAD+ + H2O → C2H4O2(acetic acid) + NADH + H+
Acetaldehyde: −127.6 kJ/mol
Acetic Acid: −389.9 kJ/mol
ΔGf2 = −389.9 + 127.6 = −262.3 kJ/mol (Exergonic)
ΣΔGf = = −262.3 + 47.2 = −215.1 kJ/mol (Exergonic, but again this does not take into consideration the reduction of NAD+.)

Step Three

C2H4O2(acetic acid) + CoA + ATP → Acetyl-CoA + AMP + PPi

(Because the Gibbs energy is a state function, we skip the formation of Acetyl-CoA (step 3), for lack of thermodynamic values.)

For the oxidation of acetic acid we have:
Acetic Acid: −389.9 kJ/mol
3H2O+2CO2: −1 500.1 kJ/mol
ΔGf4 = −1 500 + 389.6 = −1 110.5 kJ/mol (Exergonic)
ΣΔGf = = −1 110.5 - 215.1 = −1 325.6 kJ/mol (Exergonic)

Discussion of calculations

If catabolism of alcohol goes all the way to completion, then, we have a very exothermic event yielding some 1 325 kJ/mol of energy. If the reaction stops part way through the metabolic pathways, which happens because acetic acid is excreted in the urine after drinking, then not nearly as much energy can be derived from alcohol, indeed, only 215.1 kJ/mol. At the very least, the theoretical limits on energy yield are determined to be -215.1 kJ/mol to -1 325.6 kJ/mol. It is also important to note that step 1 on this reaction is endothermic, requiring 47.2 kJ/mol of alcohol, or about 3 molecules of ATP (adenosine triphosphate) per molecule of ethanol.

Organic reaction scheme

Steps of the reaction

The first three steps of the reaction pathways lead from ethanol to acetaldehyde to acetic acid to acetyl-CoA. Once acetyl-CoA is formed, it is free to enter directly into the citric acid cycle.

Gene expression and ethanol metabolism

Ethanol to acetaldehyde in human adults

In human adults, ethanol is oxidized to acetaldehyde using NAD+, mainly via the hepatic enzyme alcohol dehydrogenase IB (class I), beta polypeptide (ADH1B, EC 1.1.1.1). The gene coding for this enzyme is located on chromosome 4, locus 4q21-q23. The enzyme encoded by this gene is a member of the alcohol dehydrogenase family. Members of this enzyme family metabolize a wide variety of substrates, including ethanol, retinol, other aliphatic alcohols, hydroxysteroids, and lipid peroxidation products. This encoded protein, consisting of several homo- and heterodimers of alpha, beta, and gamma subunits, exhibits high activity for ethanol oxidation and plays a major role in ethanol catabolism. Three genes encoding alpha, beta and gamma subunits are tandemly organized in a genomic segment as a gene cluster.[9]

Ethanol to acetaldehyde in human fetuses

In human embryos and fetuses, ethanol is not metabolized via this mechanism as ADH enzymes are not yet expressed to any significant quantity in human fetal liver (the induction of ADH only starts after birth, and requires years to reach adult levels).[10] Accordingly, the fetal liver cannot metabolize ethanol or other low molecular weight xenobiotiocs. In fetuses, ethanol is instead metabolized at much slower rates by different enzymes from the cytochrome P-450 superfamily (CYP), in particular by CYP2E1. The low fetal rate of ethanol clearance is responsible for the important observation that the fetal compartment retains high levels of ethanol long after ethanol has been cleared from the maternal circulation by the adult ADH activity in the maternal liver.[11] CYP2E1 expression and activity have been detected in various human fetal tissues after the onset of organogenesis (ca 50 days of gestation).[12] Exposure to ethanol is known to promote further induction of this enzyme in fetal and adult tissues. CYP2E1 is a major contributor to the so-called Microsomal Ethanol Oxidizing System (MEOS)[13] and its activity in fetal tissues is thought to contribute significantly to the toxicity of maternal ethanol consumption.[10][14] In presence of ethanol and oxygen, CYP2E1 is known to release superoxide radicals and induce the oxidation of polyunsaturated fatty acids to toxic aldehyde products like 4-hydroxynonenal (HNE).

Acetaldehyde to acetic acid

Acetaldehyde is a highly unstable compound and quickly forms free radical structures which are highly toxic if not quenched by antioxidants such as ascorbic acid (Vitamin C) and Vitamin B1 (thiamine). These free radicals can result in damage to embryonic neural crest cells and can lead to severe birth defects. Prolonged exposure of the kidney and liver to these compounds in chronic alcoholics can lead to severe damage.[15] The literature also suggests that these toxins may have a hand in causing some of the ill effects associated with hang-overs.

The enzyme associated with the chemical transformation from acetaldehyde to acetic acid is aldehyde dehydrogenase 2 family (ALDH2, EC 1.2.1.3). In humans, the gene coding for this enzyme is found on chromosome 12, locus q24.2.[16] There is variation in this gene leading to observable differences in catalytic efficiency between people.[17]

Acetic acid to acetyl-CoA

Two enzymes are associated with the conversion of acetic acid to acetyl-CoA. The first is acyl-CoA synthetase short-chain family member 2 ACSS2 (EC 6.2.1.1).[18] The second enzyme is acetyl-CoA synthase 2 (confusingly also called ACSS1) which is localized in mitochondria.

Acetyl-CoA to water and carbon dioxide

File:Citrate.gif

Once acetyl-CoA is formed, it enters the normal citric acid cycle.

See also

References

  1. ^ ETHANOL, ACETALDEHYDE AND GASTROINTESTINAL FLORA Jyrki Tillonen ISBN 952-91-2603-4 PDF
  2. ^ group, NIH/NLM/NCBI/IEB/CDD. "NCBI CDD Conserved Protein Domain ADH_zinc_N". www.ncbi.nlm.nih.gov. Retrieved 2018-04-28. {{cite web}}: |last= has generic name (help)
  3. ^ "Fatty Acid Synthesis".
  4. ^ "Glycerolipid Metabolism".
  5. ^ "Bile Acid Biosynthesis".
  6. ^ Tanaka, F.; Shiratori, Y.; Yokosuka, O.; Imazeki, F.; Tsukada, Y.; Omata, M. (June 1997). "Polymorphism of alcohol-metabolizing genes affects drinking behavior and alcoholic liver disease in Japanese men". Alcoholism, Clinical and Experimental Research. 21 (4): 596–601. ISSN 0145-6008. PMID 9194910.
  7. ^ "Genetic polymorphisms of alcohol metabolizing enzymes". Pathol Biol (Paris). 49: 703–9. Nov 2001. PMID 11762132.
  8. ^ CRC Handbook of Chemistry and Physics, 81st Edition, 2000
  9. ^ "ADH1B alcohol dehydrogenase 1B (class I), beta polypeptide [Homo sapiens (human)] - Gene - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2018-04-28.
  10. ^ a b Ernst van Faassen and Onni Niemelä, Biochemistry of prenatal alcohol exposure, NOVA Science Publishers, New York 2011.
  11. ^ A. Nava-Ocampo et al, Reprod Toxicol 18 (2004) 613 - 617.
  12. ^ M. Brzezinski et al, J. Pharmacol Exp Therap 289 (1999) 1648 - 1653.
  13. ^ Charles Lieber, Drug Metab Rev 36 (2004) 511 - 529
  14. ^ Pregnancy and Alcohol Consumption, ed. J.D. Hoffmann, NOVA Science Publishers, New York 2011.
  15. ^ [1][dead link‍]
  16. ^ "Homo sapiens chromosome 12, reference assembly, complete sequence - Nucleotide - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2018-04-28.
  17. ^ "ALDH2 aldehyde dehydrogenase 2 family member [Homo sapiens (human)] - Gene - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2018-04-28.
  18. ^ "ACSS2 acyl-CoA synthetase short chain family member 2 [Homo sapiens (human)] - Gene - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2018-04-28.

Further reading

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